Indirect Ultra Trace Determination of Aminopolycarboxylic Acids in Surface Water Using IC-ICP/MS

Recently, organic compounds such as biomolecules and pesticides have been analyzed using the IC-ICP/MS method. This method was shown to be suitable for examining molecules of relatively small masses that are present as ions. IC-ICP/MS can also be used to monitor chelating agents, which are often used in industrial processes and have the potential to pollute the environment. Another field of application is the analysis of organophosphates in battery research (Knöll, J., 2012).

Aminopolycarboxylic acids in surface water using ion-exchange chromatography coupled to ICP/MS

Aminopolycarboxylic-acid-based chelating agents such as 1, 2-cyclohexylenedinitrilotetraacetic acid (CDTA), diethylene triamine pentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) are all agents commonly used in industrial process and household products.

These chelating agents can easily pass through the wastewater treatment plant and reach surface waters, where they are only poorly biodegradable. These agents then enter the drinking water cycle via the groundwater. Despite generally being nontoxic themselves, these chelating agents can mobilize toxic metals from the river sediments and make them biologically available.

Results

An indirect method that required only minimal sample preparation was developed for the determination of ultra-trace amounts of aminopolycarboxylic acids present in surface water. In order to convert the chelating reagents into negatively charged complexes, metal ions are added to the water sample. These complexes are then separated by ion-exchange chromatography (IC). An online coupled ICP/MS detects the complex’s central metal ion, to deduce the concentrations of the chelating agents from this. Shown in Figures 1 and 2, iron and indium were investigated as two metal ions of nominal charge +3.

Figure 1. Separation of the Fe(III) complexes of NTA, EDTA, DTPA, and CDTA in less than 15 min using column-switching.

Figure 2. Fe (III) complexes (top) and In(III) complexes (bottom) of EDTA, DTPA, and CDTA demonstrating the increased sensitivity for indium. Column 1: DV-080429-1A2 20 mm × 2 mm, PEEK, capacity 20 μmol; column 2: DV-080429-1A1 100 mm × 2 mm, PEEK, capacity 100 μmol; eluent: 20 mmol/L NH₄NO₃ (pH = 2); flow rate: 0.6 mL/min; m/z 56, 115.

In terms of sensitivity and detection limits in the lower ng/L range, In³⁺ gave the best results. Due to the low retention of the complex, the trivalent complexing agent NTA could not be accurately determined. Both iron and indium complexes of CDTA are strongly absorbed on anion exchangers and showed retention times of more than 1 hour. The retention time was decreased to less than 15 minutes by using column-switching. A sample set of 62 samples was used to validate this method. The same sample set had been previously analyzed by an external water laboratory based on the DIN EN ISO 16588 using the GC-MS technique.

Organic compounds – further applications with IC-ICP/MS

Two-dimensional ion chromatography for the separation of ionic organophosphates generated in thermally decomposed lithium hexafluorophosphate-based lithium ion battery electrolytes.

Kraft, V.; Grützke, M.; Weber, W.; Menzel, J.; Wiemers-Meyer, S.; Winter, M.; Nowak, S. (2015) J. Chrom. A 1409, 201–209

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